Hearing loss can be due to many different causes. One type of hearing loss is conductive hearing loss which occurs where the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss may often be helped by use of conventional hearing aids, which amplify sound so that acoustic information does reach the cochlea and the hair cells.
In many people who are profoundly deaf, however, the reason for deafness is sensorineural hearing loss. This type of hearing loss is due to the absence of, or destruction of, the hair cells in the cochlea which transduce acoustic signals into nerve impulses. These people are thus unable to derive suitable benefit from conventional hearing aid systems, because there is damage to or absence of the mechanism for nerve impulses to be generated from sound in the normal manner.
Cochlear implant systems have been developed for persons with sensorineural hearing loss which bypass the hair cells in the cochlea and directly deliver electrical stimulation to the auditory nerve fibres, thereby allowing the brain to perceive a hearing sensation resembling the natural hearing sensation normally delivered to the auditory nerve. U.S. Pat. No. 4,532,930, the contents of which are incorporated herein by reference, provides a description of one type of traditional cochlear implant system.
While cochlear implants have proven very successful in restoring hearing sensation to many people, persons with bilateral neural deafness are unable to benefit from such technology due to the missing transmission of electrical signals to the second neuron of the auditory pathway. Such persons are mainly patients suffering from neurofibromatosis type II and bilateral acoustic neuromas and, less frequently, patients with congenital missing auditory nerve or traumatic lesions of the auditory nerve.
Restoration of hearing to such persons is to date only possible with electrical stimulation central to the lesion site, eg. the cochlear nucleus. For example, a surface electrode plate can be placed on the surface of the cochlear nucleus in the lateral recess. While several hundred patients have now received such implants and had some restoration of hearing, the results in terms of performance are below that now achieved by cochlear implants. Postulated explanations for the results include the distortion of the anatomy at the cochlear nucleus due to the tumour size or previous treatment including gamma knife therapy, unfavourable exposure with limited visibility of the stimulation site, and the unfavourable tonotopic organisation of the nucleus with irregular frequency layer organisation in relation to the plane of the electrode plate.
With the above background in mind, there is a need to provide an implant system that provides a hearing sensation to persons unable to derive any benefit from conventional hearing aids and cochlear implant systems.
Further to this, with the benefits of electrical stimulation applied to specific parts of the brain to treat disorders such as Parkinson's Disease, Dyskinesia etc now being realised, there is a need to provide an implant system that can be easily adapted to apply such treatment.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.